11SECDWS6

iTrader: (3)

There is something that I am not understanding about exhaust flow. I’ve read recently where to get the optimum horsepower out of an engine the exhaust flow must be 2.2 cfm per horsepower. If this is true, then a 500 hp motor should need about 1100 cfm of exhaust flow to achieve its horsepower rating right?

So, if I spend a lot of money building my dual exhaust system that flows 1100 cfm, how can I utilize this exhaust system if my stage III heads flow only 500 cfm?

Adrenaline_Z

The cylinder head specs are for one port, at one depression and assuming
perfect conditions to achieve that amount of flow, etc.

The 500 CFM value only accounts for a certain duration. The flow through
the port may only be 500 CFM for a few milliseconds...and then we have to
wait for that cylinder to exhaust on the next cycle which could be another
half second depending on RPM.

There are some formulas which can get the exhaust sizes in range for
displacement and RPM. The 1100 CFM is accounting for all cylinders exhausting
in a certain time period (RPM) using an average gas volume.

11SECDWS6

iTrader: (3)

Anyone else?

I was understanding that the flow numbers for a given set of heads were of one head on a flow bench. So the flow numbers (250 cfm for my heads) were through all of the ports of one head.

This sounds about right since a 408 cid motor spinning about 6500 rpm will move about 384 cfm (192 cfm per head) of air. Below is my calculation (excluding volumetric efficiency):

(408 cid) x (6500 rpm) / [(12^3)*4] = 384 cfm

12^3 = conversion from in^3 to ft^3
4 = 4 stroke

Old SStroker

Don't get confused with port flow from a flow bench and exhaust flow on an operating engine. When you add fuel and burn the mixture, the actual exhaust flow is a lot more than the intake air flow measured on a flow bench.

I believe it was David Vizard who suggested that mufflers, when flowed at 28 in H2O, will support one hp for every 2.2 cfm they flow and not cause a restriction. This was empirically based on a number of tests he ran. It seems to prove out.

A few muffler manufacturers quote their 28 in. H20 flow numbers. Significantly, some of the more popular ones do not.

11SECDWS6

iTrader: (3)

Hmmm.....why the heck do these mufflers flow test with H2O? Its not water thats flowing through my exhaust, its air (or CO2). Big difference.

Also, I thought that when heads were flow tested, they were simply tested to the maximum flow that the head can achieve before the flow becomes turbulent and the head simply could not flow anymore regardless of how much pressure was behind it.

This all could explain why going to a bigger exhuast doesn't always net much gain (but may sound great).

Trust me, I'm not trying to be a smart ***, I'm just trying to understand. Thanks.

Old SStroker

28 inches of H2O is the pressure (or vacuum) that is used to measure AIRFLOW. They are not using water. 28 in. H2O is just about 1.0 psi.

In flowbench testing, the pressure (exhaust) or vacuum (intake) is held constant at 28 in. H2O, for example, and the valve is opened a certain amount and the flow is measured. The valve is then opened another increment, the bench adjusted to get the same 28 in. H2O and flow at that valve lift is measured.

Google is your friend. Here's a start:

http://www.cylinderheadshop.co.uk/superflow.html

Adrenaline_Z

11SEC,

If you would like I can post some pictures of a manometer and flow meter
which are typically used on a flow bench. This might help you understand
the 28 in. of water reference.

Just out of curiousity, what depression were your heads spec'd? 500 CFM is
a very good number for a SB head.

DAPSUPRSLO

iTrader: (14)

500 cfm is just about impossible on a two valve small block head in the exhaust port at 28" of depression from what i've seen. I think he had a typo or meant two ports flow a total of 500 or something?

remember when you look at flow numbers they are only to give you a general idea of what's going on. 28" just happens to be a depression which flow results coordinate best with horsepower gains/losses through years of scientific testing and experience by the many who do this stuff for a living. what's scene on the motor is vastly different from what's seen on the flow bench, but it just seems that flow numbers at 28" of depression are the best ball park figure for one to look at. this also holds true for vizard's discovery of "2.2 cfm per horsepower at 28" depression exhaust flow is required for a near zero loss exhaust system" theory. i'm sure he's established this over many years of testing and analysis.

Adrenaline_Z

I have yet to see anything close to that @ 28". I'll have to check out the
site and have a look at their test parameters for Stage III heads.

EDIT:

http://www.gmhightechperformance.com...ds/index1.html

Mid 300 CFM to 400 CFM intake; 225-240 cc intake volume; 0.600" + lift.

11SECDWS6

iTrader: (3)

Yeah, thats what I meant

So the 250 cfm is just 1 port?

11SECDWS6

iTrader: (3)

Actually, they are 250 cfm. I was thinking that this was all exhaust ports in one head. So I was thinking a total of 500 cfm exhaust for the engine. My bad

I understand perfectly the 28in of water reference now that y'all explained it.


So I take it that its very difficult to calculate the actual exhaust flow of a cylinder head in “real world conditions” meaning the heads on the engine and running?

DAPSUPRSLO

iTrader: (14)

Considering a portion of the flow that occurs through the exhaust port is sonic, mainly during low valve lifts during the initial opening of the ehxaust valve, i'd say it's very hard to calculate and gauge and make any sense out of when talking specific to exhaust port flow, ha.

11SECDWS6

iTrader: (3)

Good, I'll just stick with the 2.2 cfm per hp rule. Thats alot easier...

DAPSUPRSLO

iTrader: (14)

Ha, that's what I do too. I'm the kind of person who wants to know the "how, what, when, where, why..." of every situation but sometimes you just have to know where to draw the line!

joecar

Might be instructive to post pics.

11SECDWS6

iTrader: (3)

Thats no lie. Id love to be able to do those calcs, but I guess I'm going to have to leave it to the genius engine designers...

Adrenaline_Z

To pour salt in the wound...those flow specs don't hold up once you add
intake, exhaust, mufflers, throttle body, air filter and the list goes on.

Wouldn't it be nice to know what the entire assembly will flow through one
port having everything assembled from point A to point B?

I'll see about getting those photos posted later this evening.

RednGold86Z

Displacement * VE * RPM/2 = flow in

Flow out (in cfm etc...) = moles out (moles in combusted) (at exh temp and pressure)

It can be calculated. Just find the right equations.

11SECDWS6

iTrader: (3)

I think a good place to start is with the conservation of mass equations:

http://www.grc.nasa.gov/WWW/K-12/airplane/mflow.html

hammertime

iTrader: (1)

Actually, RednGold86Z gave you exactly what you need to determine airflow requirements.

Displacement * VE * RPM/2 = flow in
408 cu/in * .85 * 6500/2 = 1,127,100 cu/in per minute, or 652.26 cu/ft per minute for a street engine

408 cu/in * 1.10 * 6500/2 = 1,458,600 cu/in per minute, or 844.1 cu/ft per minute for a race engine

There are some other useful equations at http://www.epi-eng.com/ET-VolEff.htm

Equation 7 solves for VE, but if we input the VE we can solve for HP.

HP = (RPM x Displacement x VE) / (9411 x BSFC)
(6500 x 408 x .85) / (9411 X .45) = 532 hp

532 hp would then require 1170 cfm exhaust flow based on the 2.2 rule.

(6500 x 408 x 1.10) / (9411 X .45) = 689 hp

689 hp would then require 1515 cfm exhaust flow based on the 2.2 rule.